Multiple replication levels with pooled devices

ABSTRACT

A method for writing data to a storage pool, including receiving a first write operation for a first block, determining a first replication type for the first block, determining a number of physical blocks (n 1 ) required to write the first block to the storage pool using a size of the first block and the first replication type, if n 1  is not a multiple of the maximum supported replication level of the storage pool: allocating a number of padded physical blocks (p 1 ) to n 1  until n 1 +p 1  is a multiple of a maximum supported replication level of the storage pool, and writing the first block to the storage pool by filling in the n 1  physical blocks.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Application Ser. No.60/733,848 filed on Nov. 4, 2005, entitled “Multiple Replication Levelswith Pooled Devices” in the names of William H. Moore, Jeffrey S.Bonwick, and Matthew A. Ahrens.

The present application contains subject matter that may be related tothe subject matter in the following U.S. patent applications, which areall assigned to a common assignee: “Method and Apparatus forSelf-Validating Checksums in a File System” (application Ser. No.10/828,573) filed on Apr. 24, 2004; “Method and Apparatus for DynamicStriping” (application Ser. No. 10/828,677) filed on Apr. 21, 2004;“Method and Apparatus for Vectored Block-Level Checksum for File SystemData Integrity” (application Ser. No. 10/828,715) filed on Apr. 21,2004; “Method and Apparatus for Identifying Tampering of Data in a FileSystem” (application Ser. No. 10/853,874) filed on May 26, 2004; “Methodand System for Detecting and Correcting Data Errors Using Checksums andReplication” (application Ser. No. 10/853,837) filed on May 26, 2004;“Method and System for Detecting and Correcting Data Errors Using DataPermutations” (application Ser. No. 10/853,870) filed on May 26, 2004;“Method and Apparatus for Compressing Data in a File System”(application Ser. No. 10/853,868) filed on May 26, 2004; “Gang Blocks”(application Ser. No. 10/919,878) filed on Aug. 17, 2004; “Method andApparatus for Enabling Adaptive Endianness” (application Ser. No.10/919,886) filed on Aug. 17, 2004; “Automatic Conversion of All-ZeroData Storage Blocks into File Holes” (application Ser. No. 10/853,915)filed on May 26, 2004; “Multiple Replication Levels with Pooled Devices”(application Ser. No. 60/733,848) filed on Nov. 4, 2005; “Method andSystem for Data Replication” Ser. No. 11/434,296 filed on May 15, 2006;“Method and System Supporting Per-File and Per-Block Replication” Ser.No. 11/406,850 filed on Apr. 19, 2006; “Ditto Blocks” Ser. No.11/406,590 filed on Apr. 19, 2006; and “Method and System for AdaptiveMetadata Replication” Ser. No. 11/406,957 filed on Apr. 19, 2006.

BACKGROUND

A typical operating system includes a file system. The file systemprovides a mechanism for the storage and retrieval of files and ahierarchical directory structure for the naming of multiple files. Morespecifically, the file system stores information provided by the user(i.e., data) and information describing the characteristics of the data(i.e., metadata). The file system also provides extensive programminginterfaces to enable the creation and deletion of files, reading andwriting of files, performing seeks within a file, creating and deletingdirectories, managing directory contents, etc. In addition, the filesystem also provides management interfaces to create and delete filesystems. File systems are typically controlled and restricted byoperating system parameters. For example, most operating systems limitthe maximum number of file names that can be handled within their filesystem. Some operating systems also limit the size of files that can bemanaged under a file system.

An application, which may reside on the local system (i.e., computer) ormay be located on a remote system, uses files as an abstraction toaddress data. Conventionally, this data is stored on a storage device,such as a disk.

Data stored as files in a file system may be replicated using one ormore replication schemes. Replication schemes are typically used toenable recover data in the event of file system failures, datacorruption, etc. Data replication ensures continuous availability andprotection of data stored on disk. The follow is a non-exclusive list ofcommon replication schemes: redundant arrays of independent disks (RAID)schemes, 2-way mirroring, 3-way mirroring, etc.

SUMMARY

In general, in one aspect, the invention relates to a method for writingdata to a storage pool, comprising receiving a first write operation fora first block, determining a first replication type for the first block,determining a number of physical blocks (n1) required to write the firstblock to the storage pool using a size of the first block and the firstreplication type, if n1 is not a multiple of the maximum supportedreplication level of the storage pool: allocating a number of paddedphysical blocks (p1) to n1 until n1+p1 is a multiple of a maximumsupported replication level of the storage pool, and writing the firstblock to the storage pool by filling in the n1 physical blocks, and ifn1 is a multiple of the maximum supported replication level of thestorage pool, writing the first block to the storage pool by filling inn1 physical blocks.

In general, in one aspect, the invention relates to a system for writingdata, comprising a storage pool comprising: a plurality of child blocks,wherein each of the plurality of child blocks comprises one selectedfrom the group consisting of a data block and an indirect block, whereinthe indirect block references at least one of the plurality of childblocks, a parent block referencing at least one indirect block, and astorage pool allocator configured to store the root block and theplurality of child blocks, a file system operatively connected to thestorage pool, wherein the file system is configured to store a firstblock in the storage pool using the following method: determining afirst replication type for a first block, determining a number ofphysical blocks (n1) required to write the block to the storage poolusing a size of the first block and the first replication type, if n1 isnot a multiple of the maximum supported replication level of the storagepool: allocating a number of padded physical blocks (p1) to n1 untiln1+p1 is a multiple of a maximum supported replication level of thestorage pool, and writing the first block to the storage pool by fillingin n1 physical blocks, and if n1 is a multiple of the maximum supportedreplication level of the storage pool, writing the first block to thestorage pool by filling in n1 physical blocks, wherein the first blockis one selected from the group consisting of the child block and theparent block.

In general, in one aspect, the invention relates to a computer usablemedium comprising computer readable program code embodied therein forcausing a computer system to: receiving a first write operation for afirst block, determining a first replication type for the first block,determining a number of physical blocks (n1) required to write the firstblock to the storage pool using a size of the first block and the firstreplication type, if n1 is not a multiple of the maximum supportedreplication level of the storage pool: allocating a number of paddedphysical blocks (p1) to n1 until n1+p1 is a multiple of a maximumsupported replication level of the storage pool, and writing the firstblock to the storage pool by filling in n1 physical blocks, and if n1 isa multiple of the maximum supported replication level of the storagepool, writing the first block to the storage pool by filling in n1physical blocks.

Other aspects of the invention will be apparent from the followingdescription and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a system architecture in accordance with an embodiment ofthe invention.

FIG. 2 shows a storage pool allocator in accordance with an embodimentof the invention.

FIG. 3 shows a hierarchical data configuration in accordance with anembodiment of the invention.

FIGS. 4-5 show flow charts in accordance with an embodiment of theinvention.

FIG. 6 shows an example of replication of data in accordance with anembodiment of the invention.

FIG. 7 shows a flow chart in accordance with an embodiment of theinvention.

FIG. 8 shows a computer system in accordance with an embodiment of theinvention.

DETAILED DESCRIPTION

Specific embodiments of the invention will now be described in detailwith reference to the accompanying figures. Like elements in the variousfigures are denoted by like reference numerals for consistency. Further,the use of “ST” in the drawings is equivalent to the use of “Step” inthe detailed description below.

In the following detailed description of one or more embodiments of theinvention, numerous specific details are set forth in order to provide amore thorough understanding of the invention. However, it will beapparent to one of ordinary skill in the art that the invention may bepracticed without these specific details. In other instances, well-knownfeatures have not been described in detail to avoid obscuring theinvention.

In general, embodiments of the invention relate to data replication.More specifically, embodiments of the invention relate to enabling afile system to support multiple replication types.

FIG. 1 shows a system architecture in accordance with one embodiment ofthe invention. The system architecture includes an operating system(103) interacting with a file system (100), which in turn interfaceswith a storage pool (108). In one embodiment of the invention, the filesystem (100) includes a system call interface (102), a data managementunit (DMU) (104), and a storage pool allocator (SPA) (106).

The operating system (103) typically interfaces with the file system(100) via a system call interface (102). The operating system (103)provides operations (101) for users to access files within the filesystem (100). These operations (101) may include read, write, open,close, etc. In one embodiment of the invention, the file system (100) isan object-based file system (i.e., both data and metadata are stored asobjects). More specifically, the file system (100) includesfunctionality to store both data and corresponding metadata in thestorage pool (108). Thus, the aforementioned operations (101) providedby the operating system (103) correspond to operations on objects.

More specifically, in one embodiment of the invention, a request toperform a particular operation (101) (i.e., a transaction) is forwardedfrom the operating system (103), via the system call interface (102), tothe DMU (104). In one embodiment of the invention, the DMU (104)translates the request to perform an operation on an object directly toa request to perform a read or write operation at a physical locationwithin the storage pool (108). More specifically, the DMU (104)represents the objects as data blocks and indirect blocks as describedin FIG. 3 below. Additionally, in one embodiment of the invention, theDMU (104) includes functionality to group related work (i.e.,modifications to data blocks and indirect blocks) into I/O requests(referred to as a “transaction group”) allowing related blocks to beforwarded to the SPA (106) together. The SPA (106) receives thetransaction group from the DMU (104) and subsequently writes the blocksinto the storage pool (108). The operation of the SPA (106) is describedin FIG. 2 below.

In one embodiment of the invention, the storage pool (108) includes oneor more physical disks (disks (110A-110N)). Further, in one embodimentof the invention, the storage capacity of the storage pool (108) mayincrease and decrease dynamically as physical disks are added andremoved from the storage pool. In one embodiment of the invention, thestorage space available in the storage pool (108) is managed by the SPA(106).

FIG. 2 shows the SPA (106) in accordance with one embodiment of theinvention. The SPA (106) may include an I/O management module (200), acompression module (201), an encryption module (202), a checksum module(203), and a metaslab allocator (204). Each of these aforementionedmodules are described in detail below.

As noted above, the SPA (106) receives transactions from the DMU (104).More specifically, the I/O management module (200), within the SPA(106), receives transactions from the DMU (104) and groups thetransactions into transaction groups in accordance with one embodimentof the invention. The compression module (201) provides functionality tocompress larger logical blocks (i.e., data blocks and indirect blocks)into smaller segments, where a segment is a region of physical diskspace. For example, a logical block size of 8K bytes may be compressedto a size of 2K bytes for efficient storage. Further, in one embodimentof the invention, the encryption module (202) provides various dataencryption algorithms. The data encryption algorithms may be used, forexample, to prevent unauthorized access. In one embodiment of theinvention, the checksum module (203) includes functionality to calculatea checksum for data (i.e., data stored in a data block) and metadata(i.e., data stored in an indirect block) within the storage pool. Thechecksum may be used, for example, to ensure data has not beencorrupted.

As discussed above, the SPA (106) provides an interface to the storagepool and manages allocation of storage space within the storage pool(108). More specifically, in one embodiment of the invention, the SPA(106) uses the metaslab allocator (204) to manage the allocation ofstorage space in the storage pool (108).

In one embodiment of the invention, the storage space in the storagepool (108) is divided into contiguous regions of data, i.e., metaslabs.The metaslabs may in turn be divided into segments (i.e., portions ofthe metaslab). The segments may all be the same size, or alternatively,may be a range of sizes. The metaslab allocator (204) includesfunctionality to allocate large or small segments to store data blocksand indirect blocks. In one embodiment of the invention, allocation ofthe segments within the metaslabs is based on the size of the blockswithin the I/O requests. That is, small segments are allocated for smallblocks, while large segments are allocated for large blocks. Theallocation of segments based on the size of the blocks may allow formore efficient storage of data and metadata in the storage pool byreducing the amount of unused space within a given metaslab. Further,using large segments for large blocks may allow for more efficientaccess to data (and metadata) by reducing the number of DMU (104)translations and/or reducing the number of I/O operations. In oneembodiment of the invention, the metaslab allocator (204) may include apolicy that specifies a method to allocate segments.

As noted above, the storage pool (108) is divided into metaslabs, whichare further divided into segments. Each of the segments within themetaslab may then be used to store a data block (i.e., data) or anindirect block (i.e., metadata). FIG. 3 shows the hierarchical dataconfiguration (hereinafter referred to as a “tree”) for storing datablocks and indirect blocks within the storage pool in accordance withone embodiment of the invention. In one embodiment of the invention, thetree includes a root block (300), one or more levels of indirect blocks(302, 304, 306), and one or more data blocks (308, 310, 312, 314). Inone embodiment of the invention, the location of the root block (300) isin a particular location within the storage pool. The root block (300)typically points to subsequent indirect blocks (302, 304, and 306). Inone embodiment of the invention, indirect blocks (302, 304, and 306) maybe arrays of block pointers (e.g., 302A, 302B, etc.) that, directly orindirectly, reference to data blocks (308, 310, 312, and 314). The datablocks (308, 310, 312, and 314) contain actual data of files stored inthe storage pool. One skilled in the art will appreciate that severallayers of indirect blocks may exist between the root block (300) and thedata blocks (308, 310, 312, 314).

In contrast to the root block (300), indirect blocks and data blocks maybe located anywhere in the storage pool (108 in FIG. 1). In oneembodiment of the invention, the root block (300) and each block pointer(e.g., 302A, 302B, etc.) includes data as shown in the expanded blockpointer (302B). One skilled in the art will appreciate that data blocksdo not include this information; rather data blocks contain actual dataof files within the file system.

In one embodiment of the invention, each block pointer includes ametaslab ID (318), an offset (320) within the metaslab, a birth value(322) of the block referenced by the block pointer, and a checksum (324)of the data stored in the block (data block or indirect block)referenced by the block pointer. In one embodiment of the invention, themetaslab ID (318) and offset (320) are used to determine the location ofthe block (data block or indirect block) in the storage pool. Themetaslab ID (318) identifies a particular metaslab. More specifically,the metaslab ID (318) may identify the particular disk (within thestorage pool) upon which the metaslab resides and where in the disk themetaslab begins. The offset (320) may then be used to reference aparticular segment in the metaslab. In one embodiment of the invention,the data within the segment referenced by the particular metaslab ID(318) and offset (320) may correspond to either a data block or anindirect block. If the data corresponds to an indirect block, then themetaslab ID and offset within a block pointer in the indirect block areextracted and used to locate a subsequent data block or indirect block.The tree may be traversed in this manner to eventually retrieve arequested data block.

In one embodiment of the invention, copy-on-write transactions areperformed for every data write request to a file. Specifically, allwrite requests cause new segments to be allocated for the modified data.Therefore, the retrieved data blocks and indirect blocks are neveroverwritten (until a modified version of the data block and indirectblock is committed). More specifically, the DMU writes out all themodified data blocks in the tree to unused segments within the storagepool. Subsequently, the DMU writes out the corresponding block pointers(within indirect blocks) to unused segments in the storage pool. In oneembodiment of the invention, fields (i.e., metaslab ID, offset, birth,checksum) for the corresponding block pointers are populated by the DMUprior to sending an I/O request to the SPA. The indirect blockscontaining the block pointers are typically written one level at a time.To complete the copy-on-write transaction, the SPA issues a single writethat atomically changes the root block to reference the indirect blocksreferencing the modified data block.

Using the infrastructure shown in FIGS. 1-3, the following discussiondescribes a method for allocating blocks on disk and writing data (i.e.,data blocks) and metadata (i.e., indirect blocks) based on one or morereplication schemes. FIG. 4 shows a flow chart for writing a logicalblock (e.g., a data block or a block containing, among otherinformation, metadata) to the storage pool in accordance with oneembodiment of the invention. Those skilled in the art will appreciatethat the logical block is typically larger than the physical blocks.Initially, a request to write the logical block to the storage pool isreceived (Step 400). A determination is then made about whether thelogical block is associated with a replication policy (ST 402). Forexample, the logical block may be associated with a block level policy,a file level policy, or a file system level policy.

In one embodiment of the invention, the block level policy correspondsto a replication policy with a granularity of a block. Thus, the policyspecifies how a block is to be replicated. In one embodiment of theinvention, a file level policy corresponds to a replication policy witha granularity of a file. Thus, all blocks that are associated with agiven file are replicated in accordance with the file's replicationpolicy. In one embodiment of the invention, the file system level policycorresponds to a replication policy with a granularity of a file system.Thus, all files within the file system are replicated in accordance withthe file system's policy.

Continuing with the discussion of FIG. 4, if a block is associated witha replication policy, then the replication type (e.g., mirroring, RAID,etc.) is obtained from the replication policy (Step 404). Alternatively,if a replication policy for the logical block does not exist, then adefault replication type is determined (Step 406). Once the replicationtype is determined (using Step 404 or Step 406), the process proceeds toStep 408.

At Step 408, the number of physical blocks that need to be allocated inthe storage pool is determined using, among other information, thereplication type (Step 408). The steps involved in determining thenumber of physical blocks to allocated on disk is discussed below inFIG. 5. Once the number of physical blocks to allocate is determined,the number of physical blocks is allocated in the storage pool on aper-row basis (Step 410).

For example, if there are five disks in the storage pool and eightphysical blocks need to be allocated, then two rows are required. Inthis example, the number of rows, r, is calculated using the formular=((n1−1) div d) +1, where d is the number of disks in the storage pooland where the div operation outputs only the quotient of the divisioncalculation. Using this equation, the number of rows required to writethe eight blocks is two. The first row includes five of the eightphysical blocks and the second row includes the remaining three physicalblocks. In one embodiment of the invention, the logical block (or morespecifically the data in the logical block) is written into theallocated physical blocks column-first (Step 412). That is, althoughphysical blocks are allocated using rows, the rows are filled in on aper-column basis when the logical block is written to disk. Using theeight block example from above, physical blocks in the first threecolumns are written to prior to writing to the remaining physical blocksin columns 4 and 5.

FIG. 5 shows a flow chart showing a method for allocating blocks on diskin accordance with one embodiment of the invention. Specifically, FIG. 5describes the method corresponding to Step 408 of FIG. 4 in accordancewith one embodiment of the invention. Initially, the replication typefor the logical block is determined using the replication policy (Step500). Subsequently, the number of blocks needed to write the logicaldata to disk is computed using the size of the logical block and thereplication type (Step 502). In one embodiment of the invention, thesize of logical data corresponds to the number of physical blocksrequired to write the logical block to disk. In one embodiment of theinvention, a single logical block may correspond to more than onephysical block (e.g., data block (314) in FIG. 3 may correspond to 1K ofdata, in which case the single logical block (314) is actually twophysical blocks on disk assuming that each individual physical block is512 bytes).

In one embodiment of the invention, the number of blocks to allocate iscomputed as a function of the physical size of the data and thereplication type used to write the logical block to disk. For example,if the logical block is to be written to disk using a RAID scheme, thenthe number of physical blocks to allocate is determined by summing thenumber of physical blocks required to write the logical block into thestorage pool and an appropriate number of parity blocks (i.e., physicalblocks used to store parity data associated with one or more of thephysical blocks). Alternatively, if the size of the logical block is 2Kand the replication type is three-way mirroring, then twelve physicalblocks would be allocated in the storage pool. Those skilled in the artwill appreciate that some logical blocks in the storage pool may not bereplicated, in which case physical blocks allocated on disk wouldcorrespond directly to the size of the logical block.

At this stage, the number of physical blocks that need to be allocatedhas been determined, however, the number of physical blocks that need tobe allocated may need to be increased to prevent (or mitigate)fragmentation in file system. To determine whether the number ofphysical blocks that need to be allocated is sufficient, a determinationis made about whether the number of physical blocks determined in Step502 is a multiple of the maximum supported replication level (Step 504).If the number of physical blocks determined in Step 502 is a multiple ofthe maximum supported replication level, then the process ends. However,if the number of physical blocks determined in Step 502 is not amultiple of the maximum supported replication level, then the number ofphysical blocks determined in Step 502 is increased until the number ofphysical blocks is a multiple of the maximum supported replication level(Step 506). In one embodiment of the invention, the additional physicalblocks added in Step 506 correspond to padded physical blocks (e.g.,blocks that contain all zeros).

In one embodiment of the invention, the maximum supported replicationlevel is determined when the file system is initially configured (orsoon thereafter). Further, in one embodiment of the invention, thedefault maximum supported replication level is 2-way mirroring. Thus,the number of physical blocks determined in Step 502 (or after Step 506)must be a multiple of 2. However, if the maximum supported replicationlevel is greater than 2-way mirroring, for example, 3-way mirroring,then the number of physical blocks determined in Step 502 (or after Step506) must be a multiple of 3. Those skilled in the art will appreciatethat any maximum replication type may be used and that theaforementioned examples of maximum supported replication level are notintended to limit the scope of the invention.

FIG. 6 shows an example of using multiple replication types to writelogical blocks to the storage pool in accordance with one embodiment ofthe invention. For the purposes of the discussion of FIG. 6 assume thatthe maximum replication level is 5-way mirroring. The storage pool inFIG. 6 includes five disks (i.e., Disk 1 (630A), Disk 2 (630B), Disk 3(630C), Disk 4 (630D), Disk 5 (630E)). In the example shown in FIG. 6, alogical block (represented by M₀(600), M₁ (602), M₂ (604)) of size 1.5Kis replicated using three-way mirroring. Thus, 4.5K (or nine physicalblocks) (i.e., M₀ (600), M₁ (602), M₂ (604), M₀ (606), M₁ (608), M₂(610), M₀ (612), M₁(614), M₂ (616)) are allocated to write the logicalblock to the storage pool. As described above, because the number ofphysical blocks allocated to store 1.5K is nine, the number of physicalblocks is increased to 10 (i.e., a multiple of the maximum replicationlevel).

Further, in one embodiment of the invention, the ten blocks areallocated by row across each disk, and filled in column-first, asindicated by the order of the three-way mirrored data. That is, M₀(600), M₁ (602), M₂ (604), M₀ (606), M₁ (608), M₂ (610), M₀ (612), M₁(614), M₂ (616), and M_(FILL) (617) are filled in by column on each diskstarting with Disk 1 (630A) rather than writing the data across anentire row before proceeding to the next row.

Continuing with FIG. 6, the next logical block written to disk is 1.5Kand is written to the storage pool using a RAID scheme. In accordancewith the RAID scheme, one parity block is required (i.e., R′ (624)).Accordingly, four blocks (i.e., three data blocks (R₀ (618), R₁ (620),and R₂ (622)) and one parity block R′ (624)) are required to write thelogical block to the storage pool using the RAID scheme. However,because the maximum supported replication level is 5-way mirroring, anadditional fill block (R_(FILL) (626)) is required to be stored with thefour aforementioned physical blocks.

FIG. 6 shows how multiple files may be stored on disk using differentreplication types. In one embodiment of the invention, the replicationtype of each block stored on disk is specified in the block pointerreferencing that block. Thus, the exact location of a data or anindirect block and how the data is replicated can be obtained from theblock pointer of each block written to disk. Those skilled in the artwill appreciate that although the example shown in FIG. 6 showsthree-way mirrored data and RAID replicated data, there may be otherreplication types used to write data to disk (e.g., two-way mirroring,etc.).

Although the aforementioned description of the invention has beenfocused on writing data using various types of replication policies,those skilled in the art will appreciate that the replication type andthe number of blocks allocated to write data also affects the manner inwhich data is read and retrieved from disk. FIG. 7 shows a flow chartfor reading data in accordance with one or more embodiments of theinvention.

Initially, a transaction to read data is received (Step 700).Subsequently, the replication type, starting location (i.e., themetaslab ID and offset), and the logical block size of the next block isobtained from the root block (Step 702). That is, the block pointer ofthe root block is obtained and read to determine the location, size, andreplication type of the block referenced by the root block. The logicalblock size stored in the block pointer of a block indicates the actualsize of the data corresponding to the next block. In other words,because the logical block size of data may be different than the numberof blocks allocated to store the data (e.g., due to a replication type),the logical block size is required to determine where the data stops ondisk (i.e., how many blocks actually need to be retrieved beginning withthe starting location of the data). Next, the physical blockscorresponding to the next block are retrieved from disk (Step 704).Those skilled in the art will appreciate that not all of the physicalblocks associated with the next block need to be retrieved if the nextblock was stored using a replication scheme. In such cases, only asubset of all physical blocks corresponding to the next block need to beretrieved.

Further, those skilled in the art will appreciate that if the logicalblock is compressed, then the block pointer referencing the next blockwill also include a physical size field. The physical size fieldcorresponds to the actual size of the data stored in storage pool. Forexample, if the next block is a 4K block and is compressed to a 1K blockand stored using 2-way mirroring, then the following information isstored in the block pointer referencing the next block: (i) logicalsize=4K; (ii) physical size=1K; (iii) allocated size=2K; and (iv)replication type=2-way mirroring. From this example, those skilled inthe art will appreciate that physical size addresses compression whileallocated size address replication. Further, those skilled in the artwill appreciate that if there is no compression, then the physical sizeand the logical size are equal.

Continuing with the discussion of FIG. 7, at this stage, a determinationis made about whether the data retrieved corresponds to a data block(Step 706). If the data corresponds to a data block, then the data isextracted from the retrieved blocks and presented to the processrequesting the data (Step 708). Alternatively, if the data does notcorrespond to a data block, then the retrieved blocks correspond to anindirect block. In this case, the replication type, starting location,and the logical block size of the next block is obtained from the blockpointer in the indirect block (Step 710). Subsequently, the physicalblocks corresponding to the logical block size of the next block areretrieved from disk (Step 712). If the retrieved blocks correspond to adata block (Step 706), then the data is extracted and presented to therequesting process (Step 708). If the retrieved blocks do not correspondto a data block, then Steps 706-712 are repeated until the data block isencountered.

The invention may be implemented on virtually any type of computerregardless of the platform being used. For example, as shown in FIG. 8,a networked computer system (180) includes a processor (182), associatedmemory (184), a storage device (186), and numerous other elements andfunctionalities typical of today's computers (not shown). The networkedcomputer system (180) may also include input means, such as a keyboard(188) and a mouse (190), and output means, such as a monitor (192). Thenetworked computer system (180) is connected to a local area network(LAN) or a wide area network (e.g., the Internet) (not shown) via anetwork interface connection (not shown). Those skilled in the art willappreciate that these input and output means may take other forms.Further, those skilled in the art will appreciate that one or moreelements of the aforementioned computer (180) may be located at a remotelocation and connected to the other elements over a network. Further,the invention may be implemented on a distributed system having aplurality of nodes, where each portion of the invention (e.g., thestorage pool, the SPA, the DMU, etc.) may be located on a different nodewithin the distributed system. In one embodiment of the invention, thenode corresponds to a computer system. Alternatively, the node maycorrespond to a processor with associated physical memory.

Further, software instructions to perform embodiments of the inventionmay be stored on a computer readable medium such as a compact disc (CD),a diskette, a tape, a file, or any other computer readable storagedevice.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

What is claimed is:
 1. A method for writing data to a storage pool,comprising: receiving a first write operation for a first block;determining a first replication type for the first block; determining anumber of physical blocks (n1) required to write the first block to thestorage pool using a size of the first block and the first replicationtype; if n1 is not a multiple of a maximum supported replication levelof the storage pool: allocating a number of padded physical blocks (p1)to n1 until n1+p1 is a multiple of the maximum supported replicationlevel of the storage pool, and writing the first block to the storagepool by filling in the n1 physical blocks; and if n1 is a multiple ofthe maximum supported replication level of the storage pool, writing thefirst block to the storage pool by filling in n1 physical blocks.
 2. Themethod of claim 1, further comprising: receiving a second writeoperation for a second block; determining a second replication type forthe second block; determining a number of physical blocks (n2) requiredto write the second block to the storage pool using a size of the secondblock and the second replication type; if n2 is not a multiple of themaximum supported replication level of the storage pool: allocating anumber of padded physical blocks (p2) to n2 until n2+p2 is a multiple ofthe maximum supported replication level of the storage pool, and writingthe second block to the storage pool by filling in n2 physical blocks;and if n2 is a multiple of the maximum supported replication level ofthe storage pool, writing the second block to the storage pool byfilling in n2 physical blocks.
 3. The method of claim 2, wherein thefirst block is a data block and the second block is an indirect blockcomprising a block pointer referencing the first block.
 4. The method ofclaim 3, wherein the first replication type is stored in the secondblock, and wherein the second replication type is stored in a parentblock comprising a block pointer referencing the second block.
 5. Themethod of claim 2, wherein the first replication type is stored in afirst parent block comprising a block pointer referencing the firstblock, and wherein the second replication type is stored in a secondparent block comprising a block pointer referencing the second block. 6.The method of claim 2, wherein the first block and the second blockcorrespond to a portion of a hierarchical tree structure representing afile in a file system.
 7. The method of claim 1, wherein writing thefirst block to the storage pool by filling in n1 physical blocks,comprises: determining a number of rows (r) required to store the firstblock in the storage pool using n1; dividing the first block into aplurality of sequential blocks (s), wherein each of the plurality ofblocks is the same size as a physical block; writing a first rsequential blocks to a first disk in the storage pool; and writing ar+1^(th) sequential block to a second disk in the storage pool, whereins>r.
 8. The method of claim 7, wherein r is calculated using thefollowing formula: r=((n1−1) div d) +1, where d is the number of disksin the storage pool.
 9. A system for writing data, comprising: a storagepool comprising: a plurality of child blocks, wherein each of theplurality of child blocks comprises one selected from the groupconsisting of a data block and an indirect block, wherein the indirectblock references at least one of the plurality of child blocks; a parentblock referencing at least one indirect block; and a storage poolallocator configured to store the root block and the plurality of childblocks, a file system operatively connected to the storage pool, whereinthe file system is configured to store a first block in the storage poolusing the following method: determining a first replication type for afirst block; determining a number of physical blocks (n1) required towrite the block to the storage pool using a size of the first block andthe first replication type; if n1 is not a multiple of a maximumsupported replication level of the storage pool: allocating a number ofpadded physical blocks (p1) to n1 until n1+p1 is a multiple of themaximum supported replication level of the storage pool, and writing thefirst block to the storage pool by filling in n1 physical blocks; and ifn1 is a multiple of the maximum supported replication level of thestorage pool, writing the first block to the storage pool by filling inn1 physical blocks, wherein the first block is one selected from thegroup consisting of the child block and the parent block.
 10. The systemof claim 9, wherein the file system if further configured to store asecond block in the storage pool using the following method: receiving asecond write operation for a second block; determining a secondreplication type for the second block; determining a number of physicalblocks (n2) required to write the second block to the storage pool usinga size of the second block and the second replication type; if n2 is nota multiple of the maximum supported replication level of the storagepool: allocating a number of padded physical blocks (p2) to n2 untiln2+p2 is a multiple of the maximum supported replication level of thestorage pool, and writing the second block to the storage pool byfilling in n2 physical blocks; and if n2 is a multiple of the maximumsupported replication level of the storage pool, writing the secondblock to the storage pool by filling in n2 physical blocks, wherein thefirst block is one selected from the group consisting of the child blockand the parent block.
 11. The system of claim 10, wherein the firstblock and the second block correspond to a portion of a hierarchicaltree structure representing a file.
 12. The system of claim 10, whereinthe second replication type is stored in the first block.
 13. The systemmethod of claim 9, wherein writing the first block to the storage poolby filling in n1 physical blocks, comprises: determining a number ofrows (r) required to store the first block in the storage pool using n1;dividing the first block into a plurality of sequential blocks (s),wherein each of the plurality of blocks is the same size as a physicalblock; writing a first r sequential blocks to a first disk in thestorage pool; and writing a r+1^(th) sequential block to a second diskin the storage pool, wherein s>r.
 14. The system of claim 13, wherein ris calculated using the following formula: r=((n1−1) div d) +1, where dis the number of disks in the storage pool.
 15. A non-transitorycomputer readable medium comprising computer readable program codeembodied therein for causing a computer system to: receiving a firstwrite operation for a first block; determining a first replication typefor the first block; determining a number of physical blocks (n1)required to write the first block to the storage pool using a size ofthe first block and the first replication type; if n1 is not a multipleof a maximum supported replication level of the storage pool: allocatinga number of padded physical blocks (p1) to n1 until n1+p1 is a multipleof the maximum supported replication level of the storage pool, andwriting the first block to the storage pool by filling in n1 physicalblocks; and if n1 is a multiple of the maximum supported replicationlevel of the storage pool, writing the first block to the storage poolby filling in n1 physical blocks.
 16. The non-transitory computerreadable medium of claim 15, further comprising: receiving a secondwrite operation for a second block; determining a second replicationtype for the second block; determining a number of physical blocks (n2)required to write the second block to the storage pool using a size ofthe second block and the second replication type; if n2 is not amultiple of the maximum supported replication level of the storage pool:allocating a number of padded physical blocks (p2) to n2 until n2+p2 isa multiple of the maximum supported replication level of the storagepool, and writing the second block to the storage pool by filling in n2physical blocks; and if n2 is a multiple of the maximum supportedreplication level of the storage pool, writing the second block to thestorage pool by filling in n2 physical blocks.
 17. The non-transitorycomputer readable medium of claim 16, wherein the first block is a datablock and the second block is an indirect block comprising a blockpointer referencing the first block.
 18. The non-transitory computerreadable medium of claim 16, wherein the first block and the secondblock correspond to a portion of a hierarchical tree structurerepresenting a file in a file system.
 19. The non-transitory computerreadable medium of claim 15, wherein writing the first block to thestorage pool by filling in n1 physical blocks, comprises: determining anumber of rows (r) required to store the first block in the storage poolusing n1; dividing the first block into a plurality of sequential blocks(s), wherein each of the plurality of blocks is the same size as aphysical block; writing a first r sequential blocks to a first disk inthe storage pool; and writing a r+1^(th) sequential block to a seconddisk in the storage pool, wherein s>r.
 20. The non-transitory computerreadable medium of claim 19, wherein r is calculated using the followingformula: r=((n1−1) div d) +1, where d is the number of disks in thestorage pool.